1. Field of the Invention
The present invention relates to an underwater detection device such as a forward detection sonar, and particularly to an underwater detection device using a split beam type transducer array.
2. Description of the Related Art
A forward detection sonar is an underwater detection device for measuring the direction and distance to an obstacle and displaying pictures of the obstacle in order to prevent occurrence of a collision accident or a trouble of navigation before something happens when there is some obstacle such as a shallow place, a breakwater, a driftwood or the like ahead of a ship. Such a forward detection sonar is mounted on the hull of a ship, and transmits/receives ultrasonic waves in a 90° fan-shaped area going from the horizontal direction parallel to the sea level to the vertical direction below the ship, whereby a target in this range is detected. For example, when a breakwater exists ahead of the ship, not only the echo from the bottom of the ocean, but also the strong echo returning from the breakwater are displayed.
There are several types of forward detection sonars, but a split beam type forward detection sonar has a transducer array which is divided into two arrays. A receiving beam is formed on each array. The calculation of the coming direction of the echo, that is the direction of the target is based on the phase difference between the echoes received on both arrays and knowing the distance between the two arrays. JP-A-2006-52987 discloses a split beam type forward detection sonar.
Existence of bubbles in water induces a problem in the forward detection sonar. The bubbles are mainly generated by rotation of propellers of another ship or a motor-driven boat navigating ahead of the ship having the sonar concerned. These bubbles reflect ultrasonic waves transmitted from the forward detection sonar, and particularly when a large amount of bubbles occur at a shallow place, the signal intensity of the echo reflected from a cluster of bubbles increases. As a result, the echo based on the bottom of the ocean and the echo based on the bubble cluster appear connected to one another with the same color (for example, brown) on a display screen as shown in FIG. 11, which causes a user to misidentify the echo based on the bubble cluster as the echo of a breakwater.